Electroplating and process of producing same



Patented Nov. 1,. 1 932 UNITED STATES COLIN G. FIN'K, 01 NEW YORK, N. Y.

ELECTROPLATING AND PROCESS OF PRODUCING SAME No Drawing. Applicationfiled April 12, 1927, Serial No. 183,287. Renewed December 7, 1931.

This invention relates generally to the successful production ofelectro-plating with metals such as tungsten, thorium, molydenum,aluminum, etc., which are difiicult to 5 reduce electrolytically from anaqueous solution of their salts which may be considered available forthe purpose, partly because they have a high affinity for the oxygen ofthe water and that liberated at the anode by the breaking down of theirsalts by electrolysis when in solution in the bath, and which metals,therefore, have not been usable heretofore for plating by the usualprocess of electrolysis of an aqueous bath. The present invention moreparticularly comprises a specific adaptation of such process forobtaining a dense, substantially pure plated layer of metallic tungstendeposited from an aqueous bath on any suitable body as a cathode.

Heretofore successful electro-plating with tungsten from an aqueous bathhas not been found possible because the tungsten has not been depositedfrom any such bath in pure metallic form, but (largely because of its 5tendency to reoxidize) only as a sub-oxide (W0, or some lower oxide andsuch deposit, besides being metallically lmpure, has not adhered firmlyto the cathode base. So far as I know no substantially pure tungstenplat- 39 ing has ever been produced-prior to my present invention, andthough it has been stated in the technical literature of the art thattungsten plating has been produced by using a fused bath containing atungsten compound,

tions for the hath, no such process has ever gone into commercial use,to the best of my knowledge and belief, and my most careful repetitionsof the prior described processes 49 have not resulted in the depositionof any pure metallic tungsten. Furthermore, even if some form of fusedbath process could be made to produce pure tungsten plating, it would bevery costly in operation and commercially impracticable for a variety ofreasons. Thus, the cathode or work used in such fused bath must be onehaving a high melting point, higher than that at which the bath ismaintained. Consequently brass and other common metals andalloysdesirableganic solution, it has been frequently stated in recentpatent specifications and technical papers that aqueous baths can not beused successfully for this purpose.

In my present invention I have, general ly speaking, overcome thesedifliculties heretofore encountered in attempts to plate from an aqueousbath by the use of all or some of the following expedients. First, Iinterpose a porous diaphragm between the electrodes, preferablyimmersing in the bath a porous cup of some material insoluble in-thebath, and placing the cathode in said cup. Second, I preferably addsodium hydroxide (NaOH) or potassium hydroxide to the bath.

If then the raw material used is tun-gstic acid this causes the tungstento be present in the bath in the form of a tungstate, such as sodiumtungstate or otassium tungstate, the former, especially, ing muchmoresoluble in aqueous baths than are any of the tungsten oxides usuallyemployed as the raw material. Such tungstates may thus conveniently beformed in situ by introducin tun stic acid (W0 and sodium hy roxi e(NaOH) as the raw materials and allowing the tungstate to be produced bythe result- 35 and also by using non-aqueous organic soluing reactionsthen occurring. Third, I add some water soluble carbonaceous material(preferably dextrose, starch, sugar or some other soluble carbohydrate)to the bath, which serves as a buffer in the sodium hydroxide solutionin the well known way, decomposing gradually and thereby forming organicacids which serve to slightly reduce the original alkalinity of thebath. Fourth, I adjust the current density on the cathode at an amountwhich will produce a fairly copious evolution of hydrogen which appearsin the form of bubblesat the surface of the bath, which procedure is indirect opposition to the standard plating practice.

I may also use a minor proportion of acid in the bath, preferablyhydrochloric acid, though this is not essential. In such case the amountof acid should not be sufiicient to destroy the alkalinity of the bath.Hydrochloric acid seems to be preferable wherever a tungsten anode isused, probably because the liberated chlorine tends to preventpolarization of the anode as the chlorine ions attack it readily andevenly, much the same as in nickel plating, where hydrochloric acid isoften used to counteract polarization. The amount of such acid ofwhatever description must, however, be kept within well defined limits.As the ultimate result desired is the liberation of chlorine in thebath, this may be obtained by simply adding sodium chloride (commonsalt) to the bath, in place of hydrochloric acid. This would have theadded advantage of not disturbing the substantial alkalinity of the bathwhich it is essential to preserve.

I may secure the supply of the metal for plating by the breaking up inthe bath of a salt of the metal, as when the sodium hydroxide combineswith tungstic acid to form sodium tungstate, as above described, or thesupply may be obtained by the corrosion of an anode of the pure metal,or by both expedients.

The carbohydrate is preferably employed in excess so as to avoid thenecessity of constant renewals thereof, but the plating action goes onsatisfactorily if only a trace is present, at least when platin withtungsten.

It has been determined that the effect of the dextrose is to decreaseslightly the alkalinity of the hydroxide bath solution by its well knownaction as a buffer, producing a solution of lower pH number. Thus the pHvalue showing the hydrogen-ion concentration of a solution of 30 gramsNaOH in a litre of water was found to be about 15.6, by the antimonyelectrode method-of determination, and the showing was about the sameafter 30 grams W0 was added, but, on the addition of 30 grams ofdextrose, the H value fell to 14 and pro ressively thereafter decreaseduntil, after fi een minutes operation of the bath, it fell to 11.2. Ithas been found that the best plating action occurred when the pH valueof the bath was about 12, and that an equally good plating actionresulted when this pH value was secured by adding a suflicient furtheramount of W0 instead of adding the dextrose. Whether successful platingaction is a result of the degree of alkalinity of the bath, produced bythe above or some other method, or whether it is the result of thecreation of one or more of the very complex and somewhat indefinite lydetermined compounds difl'ering slightly from the normal tungstate, suchas the paratungstate or meta-tungstate, the presence of which is merelyindicated by a given degree of alkalinity of the bath, has not yet beendetermined.

The possible range of current density per unit of cathode surfaceimmersed is quite wide. It may be as low as 5 amperes per square foot.The upper limit rises with the alkali concentration of the bath, andwith concentrated solutions of that character may reach 200 amperes ormore. T he current density should always be sufiicient to cause a lib--eration of numerous bubbles of hydrogen at the cathode. It has generallybeen considered bad practice to produce any such considerable evolutionof hydrogen in plating with copper and other metals, not only because itinvolves a waste of current, but

also because it has a deleterious effect in that it causes thedeposition of a porous and nonadherent film of the plating metal. I havefound, however, that in latin with tungsten and similar metals of the cass to WhlCll my invention is applied'the evolution at the cathode of aconsiderable quantity of hydrogen bubbles is necessary. while the totalcurrent employed per unit of immersed cathode surface as above given maybe within the standard ranges of copper plating, the efficient current,in the usual meaning of that term, actually employed in plating, isusually much less than that employed in previous practice, the balance,perhaps ninety per cent or more, being con-. sumed in the evolution ofthe hydro en.

The porous diaphragm is particular y useful when a carbon anode is used,as the cqpious liberation of oxygen at the anode rapi decomposes thecarbon, thus fouling the bat and the cathode surface with the finelydivided carbon so produced unless this is prevented by the interpositionof the porous diaphragm.

Another observed phenomenon which, on the contrary, (and incontradiction of standard plating practice) indicates the production ofperfect plating, is the foaming of the bath in the cup about thecathode. This is apparently caused b the action of the car bohydrate onthe hy rogen bubbles, somewhat similar to the action of a soap solutionin producing soap bubbles. If the bubbles so generated in the cup aboutthe cathode are fine enough to produce a coherent layer of foam on thesurface of the bath, it indicates that hydrogen is beingliberated at theproper rate to ensure good plating.

I have found that a decided excess of caustic alkali is preferablerather than to rely on the conditions resulting from the formation ofthe normal salt, NaJVO of K WO.,.

Forming the sodium salt,Na WO in the erate quite satisfactorily.Accordingly, I

Consequently,-

prefer to operate witha bath containing frpr to ainperes per square footof cathode surface, and a temperature of bath of from 30 degrees to 90degrees centi ade. The best results are obtained with the ighertemperatures and the above described high concentration of the bathproduced by the excess of the alkali constituent.

In carrying out one example of the said improved process, I may alsoproceed as follows: In about 7 5 cubiccentimeters of water I place 75grams of sodium hydroxide (NaOH), 31 grams of the yellow oxide oftungsten (W0 60 grams of dextrose and 8 cubic centimeters ofconcentrated hydrochloric acid (I-ICl) of aspecific gravity of 1.18 andthen add sufiicient water to bring the volume up to one litre. Theresulting litre of the solution is maintained at a temperature ofdegrees centigrade and a current of 1.7 volts and 1.4 amperes passedthrough it from a tungsten anode (a treated wire slug) to a brasscathode plate having an immersed surface (including its two faces) of 5square inches, this producing a current density of 40 ampcres per squarefoot. The tungsten oxide is the commercial product of the charactertechnically known as tungstic acid, and need not be preliminarilywashed. In about 20 minutes the cathode brass plate will be 'completelycovered by a hard, gray, closely adherent deposit which has thecharacteristic appearance of pure metallic tungsten, and which I havedetermined by standard tests to consist of substantially pure tungsten.

As a further illustration of the possible modifications of the processas to the prop0r tions of the constituents of the bath I will state athird example in which I obtained a satisfactory tungsten plating on abrass plate with 6 square inches of immersed surface in about 20 minutesby using a current, density of 17.3 amperes per square foot, with avoltage of 1.9, two tungsten anodes in a litre bath containing grams ofNaOH, 200 grams of V0 60 grams of dextrose and 7.5 cubic centimeters ofHCl, maintained at a temperature of degrees centigrade.

Excellent plating has also been obtained with the second above describedbath at a temperature of 51 degrees centigrade with a current density ofonly 10 amperes per square foot of cathode plate. The bath temperaturemay be varied from 50 to degrees centigrade. the current density beingusually varied directly as the temperature. By properly regulating thecurrent density the process may be made operative at ordinary roomtemperature. For rapid working, however,

the higher degrees of temperature of the bath are, advisable and mayeven exceed the upper limit given above.

So far as I have now been able to determine,

the quantity of the acid (it used at all) must not materially exceed theproportions given in certain of the above stated examples, which arenotsuflicent to overcome the predominant alkalinity of the bath when thesodium hydroxide, or equivalent constituent is present. If hydrochloricacid exceeds 10 cubic centimeters per litr of the bath, theelectro-deposition of metallic tungsten in the bath stops, and onlyasub-oxide'is deposited. Other acids such as sulphuric, may be employedbut the best results are secured with an acid having a halogen radical,preferably hydrochloric, as

stated. Hydrofluoric acid also gives good results.

If the cathode plate is mechanically p01- ished or cleaned with acid, orboth, before immersion a perfect mirror-like lustre results in theplated tungsten surface.

During the operation of the process hydrogen .bubbles are liberatedquite rapidly at the cathode and the tungsten anode (if one is used) israpidly eaten away. A carbon anode could be substituted and the supplyof tungsten for deposition obtained entirely from the tungsten oxide inthe bath b adding proper quantities thereof from time to time, but it issimpler to replace the tungsten .by decomposition of a tungsten anode,as

this more nearly stabilizes the proportions of the bath constituents. Ifthe tungsten anode goes into solution too rapidly, a carbon anode may beused in combination with it and the submerged area of the tungsten anodeproperly adjusted to insure the desired rate of corrosion thereof. Asupplementary anode of lead may also be used, and the very smallquantity of lead which then goes into solution seems to have abeneficial effect in cleansing the tungsten film first deposited on thecathode and thus facilitating a continuous plating action so as to buildup a thicker coat of the plating. One rather peculiar phenomenon is thatwhile the first complete film of tungsten is deposited in a few minutes,subsequent building-up of the plating advances rather slowly, unless thelead, or other cathode depolarizer is used. The proport on of materialso used as a. cathode depolarizer may be so small as to be practicallynot discoverable in the plated layer, or appear only as a barelydiscoverable trace therein. When a supplementary anode of lead, or othercathode depolarizing material, is thus used the same is slowly eatenaway to form in the bath a solution of a salt of that material, and,

obviously an equivalent result could be obtained by adding such a saltdirectly to the.

bath, thus dispensing with, or supplementing the action of thesupplementary anode.

' The sodium hydroxide or the hydrochloric acid need only occasionalreplacement .in continuous operation and the dextrose, when resent inexcess, will last for a long time.

5 ther alkali metal salts may be substituted for the hydroxide. Ammoniacom ounds, however, have not so far proved satis actory, althoughammonium hydroxide can be used if very skillful mani ulation isexercised to revent its evaporat1on. Also lithium cariionate has beenfound to give excellent results. The" reason why special care must beexercised to prevent the evaporation of any volatile alkaline compoundsis, of course, that it is necessary to maintain substantial uniformityof alkalinity throughout the plating 0 eration of any alkaline bathused.

Al ali metal compounds other than the hydroxide may, however, be used,such as the carbonates.

Thus sodium carbonate (Na CO may be substituted for sodium hydroxide inmyprocess and when used has produced even better results than does thehydroxide in tungsten plating. In plating with aluminum, however, thehydroxide bath with dextrose has so far given the best results. Incarrying out the process in aluminum plating I refer to use 150 gramssodium aluminate Na A1.0.), 75 grams of dextrose and grams of sodiumhydroxide (NaOH) per litre of bath solution and electrolyze at atemperature of about 80 C. with current densities the same as in thecase of the tungsten plating.

'I have also carried out the above described process when using a carbonanode and substituting thorium oxide (ThO the material used commerciallyin making gas burner 40 mantles) and obtained a beautiful plating ofpure metallic thorium on brass. In this modification of the generalprocess the sodium hydroxide when used in the bath presumably combineswith the thorium to form a thorium compound which is much more solublethan the original thorium oxide.

As an example of my process as successfully applied in the thoriumplating, I may prepare a bath containing, per litre, 40 grams of sodiumhydroxide (NaOH) 21 grams of thorium oxide (ThO- in suspension, 60 gramsof dextrose (C l-L 0 and subject it to a current density of about 36ampcres per square foot of submerged cathode surface, at

from 3 to 5 volts, said bath being maintained at a temperature of about80 centigrade. If a trace of nickel hydrate (Ni( OH) is added the thckness of plating obtainable is increased in the manner set forth withreference to the use of a supplementary anode of lead or other cathodedepolarizer.

If the plating of pure thorium so produced is exposed to the atmospherefor any considerable period of time it changes into the 95 white oxideof thorium, but this would not occur if the plated article wereimmediately placed in a vacuum, as if used in a radio tube or ifsurrounded by an atmosphere of neutral or reducing gas.

I have also obtained a satisfactory plating of titanium by using a bathcontaining, per litre, 10 grams of titanium oxide (Ti in suspension, 15cubic centimeters of hydrofluoric acid (HF) chemically pure, or 30 cubiccentimeters of the commercial 50% solution of such acid, 10 cubiccentimeters of hydrochloric acid (H01), and 0.5 ram of gelatine. Thistitanium bath I sub ect to a current density of about 1 ampere persquare inch of submerged cathode area, at a voltage of from4 to 6 volts,maintaining the bath at a temperature of about 80 centigrade.

Plating of tungsten and similar metals can also be obtained from a bathhaving very finely divided metallic particles of the plating metalmaintained in suspension-in a dilute acid bath, such as hereinbeforesuggested. Thus by maintaining fine tungsten metal powder, such as wasformerly used in making pressed lamp filaments, in 'sus ension in adilute sulphuric acid bath (of a out 1% strength) of either copper,cobalt or nickel sulphate and subjecting the bath to a current densityof about ampere per square inch of submerged cathode surface at avoltage of from 3 to 5 volts, the bath being maintained at a temperatureof about 60 to degrees centigrade, a complete, adherent continuousplating of metallic tungsten plus a little copper, cobalt or nickel 1sobtained. While the plating so produced does not have the absolutelysmooth surface, and consequent original polish and mirror-like effect,of those produced by the preferred/forms of the invention hereinbeforedescribed, it is composed largely of tungsten and would be useful forindustrial purposes where appearance is not important, as in apparatuswhere the object is to protect the plated surfaces from the. corrosiveaction of hot gases.

In this, as in other modifications of my process, it is necessary toproperly balance the negative exponent of the hydrogen-ion concentrationof the solution represented by what is known as the pH number, all asexplained in the co-pending application Ser. No. 362,854, filed by FrankL. Jones and myself May 13, 1929'.

In operating the modifications of my process employing an alkalinebathother forms of fixed alkali salts than those previously mentionedmay, of course, by substituted, such as, potassium hydrate (KOH),lithium hydrate (LiOH) or barium hydroxide (Ba(OH) The commercialadvantages of the invention are obvious, as the materials used areplentiful and comparatively cheap, the current consumption small and themanipulation involves merely standard electro-plating practice. Littleor no artificial heat is required as compared with that employed in theoperation of the fused bath processes, and cathode bases melting ordisintegrating at low temperatures may be employed.

As this plated coating of tungsten is practically insoluble in allacids, various forms of contalners such as tanks, crucibles and similararticles used in laboratories and in chemical plants can now be made ofcheap metal bases, tungsten-plated, in place of the costly quartz,treacherous porcelain-lined apparatus, or heavy earthenware now used.

As the tungsten plating is absolutely resistant to atmosphericcorrosion, highly polished lamp reflectors for automobile and otherlamps made of it will never dim.

If the plating process can be carried "far enough, high resistanceconductors for electric heating apparatus can be made by heavily platingthin copper ribbons and then remov ing the copper by volatilization byheat, or by dissolving out with acid. Tungsten filaments can then besimilarly made by heavily plating a very fine copper wire to form a longrod, drawing this down and subsequently removing the copper by heat oracids.

Under similar conditions cutting edges to tools can be tungsten-platedand subsequently heat-treated, pen points can be tipped with tungsten inplace of iridium, contacts for ignition systems, and targets for X-rayapparatus can be made by tungsten plating in place of the costly swagedstructures of solid tungsten now required. Artistic bronze effects canbe produced by even a light plating of tungsten on a cheap metal baseand subsequently heat-treating in proper packings.

Also, as the efliciency of audions, or radio tubes, and other vacuumtubes, is now known to be dependent on the thorium present in thestandard tungsten drawn wire filament, it is obvious that the usefullife and efiiciency of such filaments may be immensely increased byplating them with thorium or other radio active metal, or by platingtungsten on a thorium base, or by building them up in alternately platedlayers of tungsten and of thorium. Heretofore it has been proposed touse thorium filaments in radio apparatus but the melting point of purethorium is so low that such filaments will not stand up at oper atingtemperatures. Evidently this difliculty can be overcome by stiffeningthe thorium body with a core or with an envelope of tungsten produced bymy improved plating process herein described.

These and other specific embodiments of the broad invention heredescribed and claimed will be specifically described and claimed inapplications to be subsequently The plating produced by the processherein described, when performed on a smooth or polished cathodesurface, has the Same smoothness as said cathode surface had beforebeing subjected to the plating o eration. Furthermore, the plated layerpro uced by my process is amorphous in structure, while all otherelectrochemically deposited metallic bodies now or heretofore producedare and have been, so far as known to me, crystalline in structure. Whythe process hereinbefore described produces an amorphous deposit insteadof one which is crystalline as in previous electrochemical platingprocesses, I have not as yet been able to determine, but the importanceof this amorphous structure of the plated layer is obvious. Such layerof amorphous material is tougher and less liable to crack and flake offthan a crystalline layer would be. Also there is an absence of theintercrystalline spaces occurring in any crystalline structure whichsometimes form pin holes throu h any such plated layer of crystalline caracter and so admit acids and water and vapors for destructive actionon the material beneath which was intended to be protected by the platedlayer.

As I employ an aqueous bath it is obvious that my process can be usedsuccessfully on any work as a cathode which will not be deformed orotherwise injuriously affected by exposure to the temperature of boilingwater.

Having described my invention, I claim 1. An electroplatingbathcomprising an aqueous solution formed by adding tungstic acid and analkali metal salt to water in proportions producing a hydrogen-ionconcentration in the resultant bath represented by a pH value of about12. v

2. An electroplating bath of predetermined alkalinity formed bypreparing aqueous so lutions containing tungstic acid, an alkali metalsalt of tungstic acid and a soluble carbohydrate.

3. An electroplating bath comprising an aqueous solution ofpredetermined alkalinity formed by introducing into water tungstic acid,an alkali metal. hydroxide and dextrose,

said bath being heated to a point above room temperature but belowboiling point.

4. An electroplating bath comprising an aqueous solution of a tungstateformed from a mixture of tungstic acid and an alkali metal hydroxide,such as sodium hydroxide, in the proportions of about 31 parts .byweight of the tungstic acid tozabout 75 parts by weight of the hydroxideper litre of the solution and to which has been added an excess of acarbohydrate soluble in water, such as dextrose.

5. An electroplating bath comprising an aqueous solution of an alkalitungstate of predetermined alkalinity equivalent to a degree ofhydrogen-ion concentration approximating that represented by the pHnumaluminum heretofore found difficult or impossible to reduce inaqueous baths, which comprises forming an aqueous bath of predeterminedalkalinity by adding to water an 4 oxide of the metal selected and analkali metal salt of such oxide and heating said bath to a temperatureof at least about degrees centigrade but below boiling point whilesubjectin it to electrolysis. v

7 if process of electroplating with metals such as tungsten, thorium,titanium and aluminum heretofore found difficult or impossible to reducein aqueous baths, which comprises forming an aqueous bath ofpredetermined alkalinity by adding to water an oxide of the metalselected and an alkali metal salt of such oxide and heating said bath toa temperature of at least about 55 degrees centigrade but below boilingpoint while subjecting it to electrolysis by an electric current havinga density of about 200 amperes per square foot of submerged cathodearea.

8. A process of electroplating with tungsten which comprises forming anaqueous bath having a degree of alkalinity approximating thatrepresented by the pH value 12, by adding tungstic acid, an alkali metalhydroxide, and a water-soluble carbohydrate, to water, and heating saidbathto a temperature of at least about 55 degrees centigrade but belowboiling oint while subjecting it to electrolysis.

9. A process of electroplating with tungsten which comprises forming anaqueous bat-h having a degree of alkalinity approximating thatrepresented by the pH value 12, by adding tungstic acid, an alkali metalhydroxide, and a water-soluble carbohydrate, to water, and heating saidbath to a temperature of at least about 55 degrees centigrade but belowboiling point while subjecting it to electrolysis by an electric currentof sufficient amperage to produce a fairly copious evolution of hydrogenbubbles at the surface of the cathode.

10. A process of electroplating with tungsten which comprises adding toan aqueous bath about 30 grams of tungstic acid and about 75 grams of analkali metal hydroxide per litre of bath volume, together withacarbohydrate susceptible of partial decomposition by said hydroxide,and subjecting said bath to electrolysis by an electric currentsufiicient to produce a fairly copious evolution of hydrogen bubbles atthe surface of the cathode.

11. The method of preparing pure metallic tungsten comprising forming anaqueous solution of a salt thereof and decomposing the same byelectricity to deposit tungsten therefrom.

12. The method of electroplating with tungsten comprising dissolving acompound thereof in an aqueous bath and passing a current between ananode and a cathode in the solution so formed while it is heated aboveroom temperature but below boiling point; whereby tungsten is lated uponthe cathode.

13. The method of forming tungsten metal comprising preparing a solutionof a tungsten compound in water at a temperature approximatin theboiling point of said solution and decomposing said compound by passingelectricity between an anode and a cathode therein; whereby a coating ofsubstantially pure tungsten is deposited on the cathode.

14. The method of electroplating with metals heretofore founddiflicultly reducible in aqueous baths, such as tungsten, thorium,titanium and aluminum, which comprises mixing an aqueous solution of anoxide of the selected metal with an alkali metal salt of such oxide alsosoluble in water and adding a carbohydrate susceptible of partialdecomposition by said alkali metal solution to reduce the alkalinity ofthe completed bath, and subjecting said bath to the action of anelectric current of suflicient ampera e to produce a fairly copiousevolution of ydrogen bubbles ,on the surface of the bath at the cathode.

15. A process for producing an adherent plated layer of tungsten havinga lustrous and smooth surface without subsequent mechanical polishing,which comprises subjecting a metallic cathode having a lustrous,polished surface to electrolytic action'in an aqueous solution of analkali tungstate of predetermined alkalinity.

16. A process for producing substantially pure plating on a suitablecathode in an electricallyconductiveaqueousbathofcontrolled, uniformalkalinity containing a metallic element of the class heretofore founddifficult to reduce in such bath such as tungsten, thorium, titanium andaluminum and a minute quantity of a metal capable of acting as a cathodedepolarizer, which process comprises sub'ecting said bath to the passageof current a justed to produce a fairly copious evolution of hydrogenbubbles at the surface thereof around' the cathode, whereby there isdeposited on said cathode a plated layer of the first mentioned metal ofany desired thickness and containing only a barel discoverable trace ofsaid depolarizing meta 17 A process for producing substantially pureplating on a suitable cathode in an electrically conductive aqueousbathof controlled, uniform alkalinity containing a metallic element of theclass heretofore found diflicult to reduce in such bath such astungsten, thorium, titanium and aluminum and a minute quantity of ametal capable of acting as a cathode depolarizer, which processcomprises subjecting said bath to the passage of current adjusted toproduce a fairly copious evolution of hydrogen bubbles at the surfacethereof around the cathode, and heating said bath to about a boilingtemperature, whereby there is deposited on said cathode a plated layerof the first mentioned metal of any desired thickness and containingonly a barely discoverable trace of said depolarizing metal.

18. A process for producing substantially pure tungsten plating, on asuitable cathode in an aqueous bath of controlled, uniform alkalinitycontaining tungsten, a minute quantity of a salt of a metal capable ofacting as a cathode depolarizer, such as lead, and an alkali metal salt,which process comprises subjecting said bath to the passage of currentadjusted to produce a fairly copious evolution of hydrogen bubbles atthe surface thereof around the cathode, whereby there is de osited onsaid cathode a plated la er of substantially pure tungsten of anydesired thickness.

19. A process for producing substantially pure tungsten plating on asuitable cathode in an aqueous bath of controlled, uniform alkalinitycontaining a minute quantity of a salt of a metal capable of acting as acathode depolarizer, such as lead, an alkali metal hydroxide, a tungstenanode and a minor proportion of a constituent having a halogen radicalto counteract polarization at said anode, which process comprisessubjecting said bath to the passage of a current adjusted to produce afairly copious evolution of hydrogen bubbles at the surface thereofaround the cathode and heating said bath to about boiling temperature,whereby there is deposited on said cathode a-plated layer ofsubstantially pure tungsten of any desired thickness.

20. The product of the herein described process, being an adherentplating of substantially pure metallic tungsten amorphous in structure.

COLIN G. FI.

